Joseph Beer in court in Mineola on Thursday, May 8, 2014. Beer is accused of being high on marijuana and driving more than 100 mph when he crashed on the Southern State Parkway on Oct. 8, 2012, killing four of his friends. Photo Credit: Howard Schnapp

The prosecution has said Beer, then 17, was speeding, driving without a license and impaired by marijuana at the time of the crash. But a defense expert testified that blood test results aren't a good measure of impairment when it comes to frequent marijuana users.